Weird worlds: Iapetus

Walnut-shaped and two-toned

(Image: NASA / JPL / SSI)

EVEN a cursory glance at Saturn’s moon Iapetus reveals it to be an oddball. It is two-toned – one half is black, the other shining white – and strangely shaped, flattened at the poles and squashed at the sides as well. A ridge runs halfway around its equator, giving it the appearance of a walnut shell.

The dark stuff on Iapetus is very black indeed, but it forms only a thin veneer less than a metre thick. It covers the moon’s leading hemisphere – the side that faces forward as it moves in its orbit – which suggests that the black material has been swept up from space as the moon moves around Saturn. This substance may originally have been ejected from the small, dark outer moons of Saturn during impacts with space debris.

Sunlight has sharpened the contrast on Iapetus by heating the dark areas so that any ice sublimates away. The water vapour then drifts around the moon, where it freezes on the colder trailing half, whitewashing it with a layer of frost.

The shape is harder to explain. Perhaps when the moon was young, molten and spinning rapidly, it was naturally distorted by its motion. If the outer layers of Iapetus froze solid at this time, some remnant of that shape might be preserved. But this theory can’t easily account for the equatorial ridge, which remains a mystery

The ingredients of Iapetus are also peculiar. Its low density implies that it is about 80 per cent ice to only 20 per cent rock, a far lighter mixture than other large moons of …

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